[PATCH V6] powerpc/bug: Add ARCH_WARN_ASM and refactor _EMIT_BUG_ENTRY for Rust support

The Rust kernel infrastructure generates inline asm for WARN() via
ARCH_WARN_ASM(file, line, flags, size), expanding it through a C
preprocessor pass (generated_arch_warn_asm.rs.S) to produce an
arch-specific asm template string for use in Rust's core::arch macros.

powerpc currently lacks ARCH_WARN_ASM and ARCH_WARN_REACHABLE, causing
Rust builds to fail on powerpc with

error: no rules expected `ARCH_WARN_ASM`
   --> /home/linkmauve/dev/linux/wii/rust/kernel/generated_arch_warn_asm.rs:1:28
    |
  1 | ::kernel::concat_literals!(ARCH_WARN_ASM("{file}", "{line}", "{flags}", "{size}"))
    |                            ^^^^^^^^^^^^^ no rules expected this token in macro call
    |
   ::: ../rust/kernel/lib.rs:279:1
    |
279 | macro_rules! concat_literals {
    | ---------------------------- when calling this macro
    |
    = note: while trying to match sequence start

error: no rules expected `ARCH_WARN_REACHABLE`
   --> /home/linkmauve/dev/linux/wii/rust/kernel/generated_arch_reachable_asm.rs:1:28
    |
  1 | ::kernel::concat_literals!(ARCH_WARN_REACHABLE)
    |                            ^^^^^^^^^^^^^^^^^^^ no rules expected this token in macro call
    |
   ::: ../rust/kernel/lib.rs:279:1
    |
279 | macro_rules! concat_literals {
    | ---------------------------- when calling this macro
    |
    = note: while trying to match sequence start

error: aborting due to 2 previous errors

To add ARCH_WARN_ASM, _EMIT_BUG_ENTRY first needs to be refactored.
The old definition was a bare macro with no parameters, relying on
positional asm operand references (%0-%3), hardcoding the backward
reference to local label 1b, and including .org/.previous directives
inline. That made it impossible to compose as a plain string outside of
an asm operand context, and left an invisible contract that callers must
always emit their trap at label 1:.

Refactor _EMIT_BUG_ENTRY to take explicit (bug_entry, trap, file, line, flags)
string arguments via string concatenation. This removes the dependency
on asm operand numbering and makes the labels an explicit argument,
so the caller's intent is visible at the call site and a future caller
using a different label cannot silently produce a wrong bug table entry.

Move the .org and .previous directives out of _EMIT_BUG_ENTRY and into
each call site, so BUG_ENTRY() can still pass sizeof(struct bug_entry)
as an asm operand while ARCH_WARN_ASM can supply its own size string
independently.

Add ARCH_WARN_REACHABLE as an empty define, matching the arm64
convention, indicating that no additional reachability annotation is
needed after a WARN on powerpc.

Reported-by: FUJITA Tomonori <redacted>
Closes: https://lore.kernel.org/all/anG67Q6Y59kDqh-c@desktop (local)
Fixes: 73b741adb264 ("rust: Add PowerPC support")
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <redacted>
---
Changelog:
V5 -> V6:
 - Dropped KUit tests from this series. It will be sent separately.
V5: https://lore.kernel.org/all/20260915090453.1227034-1-mkchauras@gmail.com/ (local)

V4 -> V5:
 - Fixed a build error with DEBUG_BUGVERBOSE=n
 - Added a label for bug entry
V4: https://lore.kernel.org/all/20260912065902.24017-1-mkchauras@gmail.com/ (local)

V3 -> V4:
 - Fix Label with appending b at end
 - Add KUnit test patch
 - Tested on ppc64le pseries: pass:5 fail:0 skip:0
 - Tested on ppc32 QEMU mac99 G4: pass:5 fail:0 skip:0
 - Tested on ppc64le QEMU pseries: pass:5 fail:0 skip:0
V3: https://lore.kernel.org/all/20260910100801.2159785-2-mkchauras@gmail.com (local)

V2 -> V3:
 - Add label argument in _EMIT_BUG_ENTRY
V2: https://lore.kernel.org/all/20260910071252.1950488-2-mkchauras@gmail.com (local)

V1 -> V2:
 - commit message now has error, fixes tag and closes tag
V1: https://lore.kernel.org/all/20260819084825.969116-1-mkchauras@gmail.com (local)

 arch/powerpc/include/asm/bug.h | 46 ++++++++++++++++++----------------
 1 file changed, 25 insertions(+), 21 deletions(-)

diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
index 0db48977c70c..bf31ee1e902a 100644
--- a/arch/powerpc/include/asm/bug.h
+++ b/arch/powerpc/include/asm/bug.h

@@ -32,34 +32,38 @@
 #endif /* verbose */
 
 #else /* !__ASSEMBLER__ */
-/* _EMIT_BUG_ENTRY expects args %0,%1,%2,%3 to be FILE, LINE, flags and
-   sizeof(struct bug_entry), respectively */
 #ifdef CONFIG_DEBUG_BUGVERBOSE
-#define _EMIT_BUG_ENTRY				\
-	".section __bug_table,\"aw\"\n"		\
-	"2:	.4byte 1b - .\n"		\
-	"	.4byte %0 - .\n"		\
-	"	.short %1, %2\n"		\
-	".org 2b+%3\n"				\
-	".previous\n"
+#define _EMIT_BUG_ENTRY(bug_entry, trap, file, line, flags)	\
+	".section __bug_table,\"aw\"\n"				\
+	#bug_entry ":	.4byte " #trap " - .\n"			\
+	"		.4byte " file " - .\n"			\
+	"		.short " line ", " flags "\n"
 #else
-#define _EMIT_BUG_ENTRY				\
-	".section __bug_table,\"aw\"\n"		\
-	"2:	.4byte 1b - .\n"		\
-	"	.short %2\n"			\
-	".org 2b+%3\n"				\
-	".previous\n"
+#define _EMIT_BUG_ENTRY(bug_entry, trap, file, line, flags)	\
+	".section __bug_table,\"aw\"\n"				\
+	#bug_entry ":	.4byte " #trap " - .\n"			\
+	"		.short " flags "\n"
 #endif
 
-#define BUG_ENTRY(cond_str, insn, flags, ...)		\
-	__asm__ __volatile__(				\
-		"1:	" insn "\n"			\
-		_EMIT_BUG_ENTRY				\
+#define BUG_ENTRY(cond_str, insn, flags, ...)			\
+	__asm__ __volatile__(					\
+		"1:	" insn "\n"				\
+		_EMIT_BUG_ENTRY(2, 1b, "%0", "%1", "%2")	\
+		".org 2b+%3\n"					\
+		".previous\n"					\
 		: : "i" (WARN_CONDITION_STR(cond_str) __FILE__), "i" (__LINE__),	\
-		  "i" (flags),				\
-		  "i" (sizeof(struct bug_entry)),	\
+		  "i" (flags),					\
+		  "i" (sizeof(struct bug_entry)),		\
 		  ##__VA_ARGS__)
 
+#define ARCH_WARN_ASM(file, line, flags, size)			\
+		"1:	twi 31, 0, 0\n"				\
+		_EMIT_BUG_ENTRY(2, 1b, file, line, flags)	\
+		".org 2b+" size "\n"				\
+		".previous\n"
+
+#define ARCH_WARN_REACHABLE
+
 /*
  * BUG_ON() and WARN_ON() do their best to cooperate with compile-time
  * optimisations. However depending on the complexity of the condition

-- 
2.55.0